Interpretive Summary: Emerging or newly discovered viruses are becoming more of a threat to both human and animal health. Some of these viruses belong to the calicivirus family of viruses. Two groups of caliciviruses, human enteric calicivirus (HECV) and porcine enteric calicivirus (PECV), do not grow in cell cultures. The HECV are a major cause of viral food poisoning. There is evidence that PECV may be involved in diarrhea outbreaks in swine herds. The inability to grow these viruses in the laboratory makes them difficult to study. To better understand the immune response against caliciviruses in general, feline caliciviruses that grow well in the laboratory were studied. We examined portions of the virus that antibodies are made against in the host animal following an infection. We found that changes in the calicivirus coat protein are responsible for altered antibody recognition of these viruses. Further research efforts can now be directed toward understanding these changes for a clearer knowledge of how antibodies are effective in inactivation of calicivirus particles. This information will allow researchers to understand immune responses against the caliciviruses and make useful veterinary and human vaccines possible.

Technical Abstract:
Feline calicivirus (FCV) strains can show significant antigenic variation when tested for cross-reactivity with antisera produced against other FCV strains. Previous work has demonstrated the presence of hypervariable amino acid sequences in the capsid protein of FCV (designated regions C and E) that were postulated to constitute the major antigenic determinants of the virus. To examine the involvement of hypervariable sequences in determining the antigenic phenotype, the nucleotide sequences encoding the E regions from three antigenically distinct parental FCV strains (CFI, KCD and NADC) were exchanged for the equivalent sequences in an FCV URB infectious cDNA clone. Two of the three constructs were recovered as viable, chimeric viruses. In six additional constructs, of which three were recovered as viable virus, the E region from the parental viruses was divided into "left" and "right" halves and engineered into the infectious clone. A final viable construct contained the C, D and E regions of the NADC parental strain. Recovered chimeric viruses showed considerable antigenic variation from the parental viruses when tested against parental hyperimmune serum. No domain exchange was able to confer complete recognition by parental antiserum with the exception of the KCD E region exchange, which was neutralized at a near homologous titer with KCD antiserum. These data demonstrate that it is possible to recover engineered chimeric FCV strains that possess altered antigenic characteristics. Furthermore, the E hypervariable region of the capsid protein appears to play a major role in the formation of the antigenic structure of the virion where conformational epitopes may be more important than linear in viral neutralization.